// convert key to a byte array int len = key.Text.Length / 2; if (len == 0) throw new ApplicationException("key must be provided"); byte[] k = new byte[len]; for (int i = 0; i < len; i++) { k[i] = Convert.ToByte(key.Text.Substring(i * 2, 2), 16); } summaryText += "key=" + key.Text.Trim() + "\r\n\r\n";